Optical waveguide for directional backlight

ABSTRACT

A switchable directional backlight for a privacy display comprises a waveguide with first and second opposing input ends and a turning film arranged to collect light output from the waveguide for input into a spatial light modulator. The waveguide has an array of light deflecting features arranged on one guiding surface and an opposing planar surface. Light deflecting features are arranged such that light input from the first input end is output with a narrow angular range and light input from the second input end is output with a wide angular range.

TECHNICAL FIELD

This disclosure generally relates to illumination from light modulationdevices, and more specifically relates to optical stacks for providingcontrol of illumination for use in display including privacy display,outdoors display, low stray light displays for night-time use and powersaving displays.

BACKGROUND

Privacy displays provide image visibility to a primary user that istypically in an on-axis position and reduced visibility of image contentto a snooper, that is typically in an off-axis position. A privacyfunction may be provided by micro-louvre optical films that transmit ahigh luminance from a display in an on-axis direction with low luminancein off-axis positions, however such films are not switchable, and thusthe display is limited to privacy only function.

Switchable privacy displays may be provided by control of the off-axisoptical output.

Control may be provided by means of luminance reduction, for example bymeans of switchable backlights for a liquid crystal display (LCD)spatial light modulator. Display backlights in general employ waveguidesand edge emitting sources. Certain imaging directional backlights havethe additional capability of directing the illumination through adisplay panel into viewing windows. An imaging system may be formedbetween multiple sources and the respective window images. One exampleof an imaging directional backlight is an optical valve that may employa folded optical system and hence may also be an example of a foldedimaging directional backlight. Light may propagate substantially withoutloss in one direction through the optical valve whilecounter-propagating light may be extracted by reflection off tiltedfacets as described in U.S. Pat. No. 9,519,153, which is hereinincorporated by reference in its entirety.

Control of off-axis privacy may further be provided by means of contrastreduction, for example by adjusting the liquid crystal bias tilt in anIn-Plane-Switching LCD.

BRIEF SUMMARY

According to the present disclosure, a directional illuminationapparatus may comprise an optical waveguide with a microstructured lightguiding surface and a planar light guiding surface that are arranged toprovide a narrow light output cone for light input at one input end. Themicrostructured light guiding surface is arranged such that the opticaloutput may be switchable by means of control of first and second lightsources arranged at opposing input ends of the optical waveguide. Abacklight may comprise the optical waveguide, a light turning film and arear reflector. A display may comprise the backlight and a spatial lightmodulator. The display may further comprise a switchable retardationoptical stack comprising a switchable liquid crystal layer, compensationretarder and an absorptive polariser.

In a wide angle mode of operation, the switchable liquid crystal layeris switched to provide high transmission to ambient illumination for awide range of polar output angles and the backlight is arranged toprovide light with a wide field of view from light input at the secondend. In a privacy mode of operation, the switchable liquid crystal layeris switched to provide high transmission to ambient illumination indirections observed by a primary user; and low transmission indirections observed by a snooper and the backlight is arranged toprovide a narrow field of view from light input at the first end.

According to a first aspect of the present disclosure, there is providedan optical waveguide comprising: first and second opposed light guidingsurfaces for guiding light along the optical waveguide by total internalreflection; and at least one light input end arranged between the firstand second light guiding surfaces, wherein the light input end extendsin a lateral direction, wherein the second light guiding surfacecomprises: (i) a plurality of non-inclined light extraction featuresarranged in an array, each non-inclined light extraction feature beingelongate, extending in a longitudinal direction perpendicular to thelateral direction, and having surface normal directions that vary in aplane orthogonal to the longitudinal direction and that have nocomponent of tilt in the longitudinal direction; and (ii) a plurality ofinclined light extraction features arranged in an array, each inclinedlight extraction feature having a surface normal direction with a tiltthat has a component in the longitudinal direction, the plurality ofnon-inclined light extraction features and the plurality of inclinedlight extraction features being oriented to direct guided light throughthe first and second light guiding surfaces as output light.

Advantageously a single moulding surface may provide the opticalstructures of an optical waveguide. Light extraction may be from theoptical waveguide towards a display, increasing extraction efficiency.The optical waveguide may have no optical coatings, increasingefficiency and reducing cost and complexity. A very thin waveguide maybe arranged and may be formed on or in a flexible substrate to provide aflexible display.

The at least one light input end may comprise a first light input endand a second light input end facing the first light input end in thelongitudinal direction. The optical output from the waveguide may beprovided with first and second angular illumination profiles for lightinput from the first and second end respectively. The optical output ofthe optical waveguide may be different for light input at the firstlight input end and the second input end. Advantageously a switchablebacklight maybe provided to achieve switching between privacy and wideangle mode of a display apparatus.

The plurality of non-inclined light extraction features may comprise alenticular surface or an elongate prismatic surface. Advantageously thefeatures may be tooled by known tooling methods and may be formed on orin the substrate of the optical waveguide with low cost and highuniformity.

The plurality of inclined light extraction features may comprise a firstplurality of inclined light extraction features, each light extractionfeature of the first plurality of inclined light extraction featureshaving a surface normal direction that has a tilt with a component inthe longitudinal direction that is away from the first light input end;and a second plurality of inclined light extraction features, each lightextraction feature of the second plurality of inclined light extractionfeatures having a surface normal direction that has a tilt with acomponent in the longitudinal direction that is towards the first lightinput end. The magnitude of the component in the longitudinal directionof the tilt of the surface normal direction of the first plurality ofinclined light extraction features may be different from the magnitudeof the component in the longitudinal direction of the tilt of thesurface normal direction of the second plurality of inclined lightextraction features.

The component in the longitudinal direction of the tilt of the surfacenormal direction of the first plurality of inclined light extractionfeatures may be between 0.25 degrees and 5 degrees, preferably between0.5 degrees and 4 degrees and most preferably between 1 degree and 3degrees. Advantageously a uniform optical output may be achieved with anarrow cone angle.

The component in the longitudinal direction of the tilt of the surfacenormal direction of the second plurality of inclined light extractionfeatures may be between 80 degrees and 90 degrees, and preferablybetween 85 degrees and 90 degrees. Advantageously a uniform opticaloutput may be achieved with a wide cone angle.

The inclined light extraction features may comprise planar inclinedlight extraction features. The planar inclined light extraction featuresmay have surface normal directions that have no component in the lateraldirection. The inclined light extraction features may compriselenticular surfaces that are extended in the longitudinal direction.Advantageously the features may be tooled by known tooling methods andmay be formed on or in the substrate of the optical waveguide with lowcost and high uniformity. Further the light output cones may be arrangedto provide output collimated light.

At least some of the plurality of non-inclined light extraction featuresmay be intersected by at least one inclined light extraction feature.The plurality of non-inclined light extraction features that areintersected by at least one first inclined light extraction feature maybe intersected by the at least one first inclined light extractionfeature in an intersection region; and the width of the non-inclinedlight extraction feature in the intersection region may vary in thelongitudinal direction. Advantageously the non-inclined and inclinedlight extraction features may be conveniently tooled on the same tooland replicated with low cost and high fidelity.

The arrays may be two-dimensional. Advantageously uniform illuminationof a spatial light modulator may be provided to achieve a directionalbacklight.

The first light guiding surface may comprise a planar surface.

According to a second aspect of the present disclosure there is provideda backlight apparatus comprising the optical waveguide of the firstaspect and at least one light source arranged to input light into theoptical waveguide at the at least one input end. Advantageously abacklight with a narrow output cone angle may be provided.

At least one light source may be arranged to input light into theoptical waveguide at the first input end, and at least one further lightsource may be arranged to input light into the optical waveguide at thesecond input end. Advantageously a backlight may be provided that hasfirst and second angular output profiles when the at least one lightinput end comprises a first light input end and a second light input endfacing the first light input end in the longitudinal direction.

The backlight apparatus may further comprise a control system arrangedto control the luminous flux from the first and second light sources.Advantageously the output field of view of the backlight may becontrolled. The backlight may switch between a narrow angle profile, awide angle profile and intermediate profiles that are the combination ofnarrow and wide angle profiles.

The backlight apparatus may further comprise a light turning filmcomprising an array of prismatic elements that are elongate in thelateral direction. Advantageously at least some of the output of thedisplay may be towards the normal direction of a display. At least someof the light may be directed to an on-axis observer.

The second surface of the optical waveguide may be arranged between thefirst surface of the optical waveguide and the light turning film; andthe light turning film may be arranged to receive light transmittedthrough the second surface of the optical waveguide. Advantageouslyefficiency of light output may be increased in comparison toarrangements in which a rear reflector receives light transmittedthrough the second surface of the optical waveguide.

The prismatic elements of the array of prismatic elements may eachcomprise opposing first and second prismatic faces wherein: each firstprismatic face has a surface normal direction that has a component thatis inclined in the longitudinal direction towards the first input end;and each second prismatic face has a surface normal direction that has acomponent that is inclined in the longitudinal direction away from thefirst input end. Advantageously the size of the output cone angle may beincreased.

Each first prismatic face comprises a planar surface and each secondprismatic face comprises an undulating surface; wherein when light isinput into the first input end of the optical waveguide, output lightfrom the optical waveguide is refracted by the second prismatic facetand is reflected by the first prismatic face; and when light is inputinto the second input end of the optical waveguide, output light fromthe optical waveguide is refracted by the first prismatic facet and isreflected by the second prismatic face.

Advantageously the light cone for light from the first light source mayhave small diffusion and the light cone for light from the second lightsource may have a larger diffusion. The wide angle mode light cone maybe increased in size, whereas the narrow cone angle profile may havesimilar size. Advantageously wide angle visibility is increased, andprivacy visibility is not significantly degraded.

The backlight apparatus may further comprise a rear reflector facing thefirst light guiding surface that is arranged to reflect lighttransmitted through the first surface of the optical waveguide.Advantageously light output efficiency may be increased.

According to a third aspect of the present disclosure there is provideda display apparatus comprising the backlight apparatus of the secondaspect and further comprising a spatial light modulator arranged toreceive light from the light turning film. Advantageously a display maybe provided with high efficiency of optical output for an on-axisobserver.

The display apparatus may comprise at least one display polariserarranged on a side of the spatial light modulator; an additionalpolariser arranged on the same side of the spatial light modulator asthe display polariser; and a switchable liquid crystal retardercomprising a layer of liquid crystal material arranged between thedisplay polariser and the additional polariser. When a light source isarranged to input light into the optical waveguide at the first inputend, a first voltage may be applied across the switchable liquid crystalretarder and when a light source is arranged to input light into theoptical waveguide at the second input end, a second voltage different tothe first voltage may be applied across the switchable liquid crystalretarder.

Advantageously the off-axis luminance for a snooper may be reduced andprivacy mode performance enhanced. The on-axis efficiency may besubstantially unchanged. Further the wide angle mode performance will beincreased.

Any of the aspects of the present disclosure may be applied in anycombination.

Embodiments of the present disclosure may be used in a variety ofoptical systems. The embodiments may include or work with a variety ofprojectors, projection systems, optical components, displays,microdisplays, computer systems, processors, self-contained projectorsystems, visual and/or audiovisual systems and electrical and/or opticaldevices. Aspects of the present disclosure may be used with practicallyany apparatus related to optical and electrical devices, opticalsystems, presentation systems or any apparatus that may contain any typeof optical system. Accordingly, embodiments of the present disclosuremay be employed in optical systems, devices used in visual and/oroptical presentations, visual peripherals and so on and in a number ofcomputing environments.

Before proceeding to the disclosed embodiments in detail, it should beunderstood that the disclosure is not limited in its application orcreation to the details of the particular arrangements shown, becausethe disclosure is capable of other embodiments. Moreover, aspects of thedisclosure may be set forth in different combinations and arrangementsto define embodiments unique in their own right. Also, the terminologyused herein is for the purpose of description and not of limitation.

These and other advantages and features of the present disclosure willbecome apparent to those of ordinary skill in the art upon reading thisdisclosure in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example in the accompanyingFIGURES, in which like reference numbers indicate similar parts, and inwhich:

FIG. 1 is a schematic diagram illustrating in side perspective view anoptical stack of a switchable privacy display device comprising aswitchable backlight arranged to illuminate a spatial light modulator;

FIG. 2 is a schematic diagram illustrating in side perspective view anoptical waveguide comprising a non-inclined lenticular surface andinclined planar surfaces;

FIG. 3A and FIG. 3B are schematic diagrams illustrating in perspectiveviews an optical waveguide comprising a non-inclined lenticular surfaceand inclined planar surfaces;

FIG. 4A is a schematic diagram illustrating in side perspective view anon-inclined lenticular surface of an optical waveguide not in anintersection region;

FIG. 4B is a schematic diagram illustrating in side perspective view anon-inclined lenticular surface of an optical waveguide in anintersection region;

FIG. 4C is a schematic diagram illustrating in side perspective view afirst inclined planar region of an optical waveguide in the intersectionregion;

FIG. 4D is a schematic diagram illustrating in side perspective view thetilted cross sectional profile of the first inclined planar region of anoptical waveguide in the intersection region;

FIG. 4E is a schematic diagram illustrating in side perspective view asecond inclined planar region of an optical waveguide in an intersectionregion;

FIG. 4F is a schematic diagram illustrating in side view a non-inclinedlenticular surface of an optical waveguide;

FIG. 4G is a schematic diagram illustrating in top view a non-inclinedlenticular surface of an optical waveguide and a first inclined planarregion of an optical waveguide in the intersection region;

FIG. 5A is a schematic diagram illustrating in side view operation of afirst inclined planar region of an optical waveguide comprising a planarnon-inclined region for on-axis illumination;

FIG. 5B is a schematic diagram illustrating in side view operation of anon-inclined lenticular structure for on-axis illumination;

FIG. 6A is a schematic diagram illustrating in top view operation of anon-inclined lenticular structure for off-axis illumination;

FIG. 6B is a schematic diagram illustrating in end view operation of anon-inclined lenticular structure for off-axis illumination;

FIG. 6C is a schematic diagram illustrating in side view operation of anon-inclined lenticular structure for off-axis illumination;

FIG. 6D is a schematic diagram illustrating in top view operation of aninclined planar feature for off-axis illumination;

FIG. 6E is a schematic diagram illustrating in end view operation of aninclined planar feature for off-axis illumination;

FIG. 6F is a schematic diagram illustrating in side view operation of aninclined planar feature for off-axis illumination;

FIG. 6G is a schematic diagram illustrating in top view operation of anon-inclined lenticular structure for off-axis illumination afterincidence with an inclined planar feature;

FIG. 6H is a schematic diagram illustrating in end view operation of anon-inclined lenticular structure for off-axis illumination afterincidence with an inclined planar feature;

FIG. 6I is a schematic diagram illustrating in side view operation of anon-inclined lenticular structure for off-axis illumination afterincidence with an inclined planar feature;

FIG. 7 is a schematic diagram illustrating in top view output of anoptical waveguide;

FIG. 8A is a schematic graph illustrating an iso-luminance field-of-viewpolar plot for the arrangement of FIG. 7 after incidence on a lightturning film;

FIG. 8B is a schematic diagram illustrating in perspective view a lightturning film comprising planar opposing faces;

FIG. 9A is a schematic diagram illustrating in top view operation of aninclined planar feature for light from the second input end;

FIG. 9B is a schematic diagram illustrating in end view operation of aninclined planar feature for light from the second input end;

FIG. 9C is a schematic diagram illustrating in side view operation of aninclined planar feature for light from the second input end;

FIG. 10 is a schematic diagram illustrating in side view a backlightcomprising an optical waveguide comprising a non-inclined lenticularsurface and inclined planar surfaces, a rear reflector and a lightturning film;

FIG. 11A, FIG. 11B, FIG. 11C, and FIG. 11D are schematic diagramsillustrating in perspective views a light turning film wherein eachfirst prismatic face comprises a planar surface and each secondprismatic face comprises an undulating surface;

FIG. 12A is a schematic graph illustrating iso-luminance field-of-viewpolar plots for different positions across a backlight comprising anoptical waveguide of FIG. 2 and light turning film of FIG. 8B when lightis input into the first end of the optical waveguide;

FIG. 12B is a schematic graph illustrating iso-luminance field-of-viewpolar plots for different positions across a backlight comprising anoptical waveguide of FIG. 2 and light turning film of FIGS. 11A-1ID whenlight is input into the first end of the optical waveguide;

FIG. 12C is a schematic graph illustrating iso-luminance field-of-viewpolar plots for different positions across a backlight comprising anoptical waveguide of FIG. 2 and light turning film of FIG. 8B when lightis input into the second end of the optical waveguide;

FIG. 12D is a schematic graph illustrating iso-luminance field-of-viewpolar plots for different positions across a backlight comprising anoptical waveguide of FIG. 2 and light turning film of FIGS. 11A-11D whenlight is input into the second end of the optical waveguide;

FIG. 13A is a schematic graph illustrating angular luminance profilesfor light input from first and second input ends of an opticalwaveguide;

FIG. 13B is a schematic graph illustrating normalised angular luminanceprofiles for light input from first and second input ends of an opticalwaveguide;

FIG. 14A, FIG. 14B, FIG. 14C, FIG. 14D, and FIG. 14E are schematicdiagrams illustrating in perspective views a method to form a tool forfabrication of a surface of an optical waveguide comprising non-inclinedlenticular surface and inclined planar surfaces;

FIG. 14F and FIG. 14G are schematic diagrams illustrating in perspectiveviews tools for fabrication of a surface of an optical waveguidecomprising non-inclined lenticular surface and inclined planar surfaces;

FIG. 15 is a schematic diagram illustrating in top view a curved inputshape optical waveguide, in accordance with the present disclosure.

FIG. 16 is a schematic diagram illustrating in a first perspective viewa light turning film comprising a planar opposing face and a kinkedopposing face;

FIG. 17 is a schematic diagram illustrating in a second perspective viewa light turning film comprising a planar opposing face and a kinkedopposing face and a planar output surface;

FIG. 18A is a schematic graph illustrating iso-luminance field-of-viewpolar plots for different positions across a backlight comprising anoptical waveguide of FIG. 2 and light turning film of FIGS. 17A-B whenlight is input into the first end of the optical waveguide;

FIG. 18B is a schematic graph illustrating iso-luminance field-of-viewpolar plots for different positions across a backlight comprising anoptical waveguide of FIG. 2 and light turning film of FIGS. 17A-B whenlight is input into the second end of the optical waveguide;

FIG. 19A is a schematic diagram illustrating in top view the opticalstack of a privacy display comprising an optical waveguide arranged toprovide privacy mode illumination and a switchable liquid crystalretarder;

FIG. 19B is a schematic graph illustrating an iso-transmissionfield-of-view polar plot for a switchable retarder in a privacy mode ofoperation;

FIG. 20A is a schematic diagram illustrating in top view the opticalstack of a privacy display comprising an optical waveguide arranged toprovide wide angle illumination and a switchable liquid crystalretarder;

FIG. 20B is a schematic graph illustrating an iso-transmissionfield-of-view polar plot for a switchable retarder in wide angle mode ofoperation;

FIG. 21A is a schematic diagram illustrating in front perspective viewobservation of transmitted output light for a display operating inprivacy mode;

FIG. 21B is a schematic diagram illustrating in front perspective viewsthe appearance of the display of FIGS. 1A-1C operating in privacy mode;

FIG. 22 and FIG. 23 are schematic diagrams illustrating in sideperspective views an optical waveguide comprising a non-inclinedlenticular surface and inclined planar surfaces that do not intersectthe non-inclined lenticular surfaces;

FIG. 24A is a schematic diagram illustrating in side perspective view anon-inclined lenticular surface of an optical waveguide;

FIG. 24B is a schematic diagram illustrating in side perspective view afirst inclined planar region of an optical waveguide;

FIG. 24C is a schematic diagram illustrating in side perspective view asecond inclined planar region of an optical waveguide;

FIG. 25 is a schematic diagram illustrating in side perspective view anoptical waveguide comprising a non-inclined elongate prismatic surfaceand first and second inclined intersecting planar surfaces;

FIG. 26A is a schematic graph illustrating iso-luminance field-of-viewpolar plots for different positions across a backlight comprising anoptical waveguide of FIG. 25 and light turning film of FIGS. 11A-11Dwhen light is input into the first end of the optical waveguide;

FIG. 26B is a schematic graph illustrating iso-luminance field-of-viewpolar plots for different positions across a backlight comprising anoptical waveguide of FIG. 25 and light turning film of FIGS. 11A-11Dwhen light is input into the second end of the optical waveguide;

FIG. 27A, FIG. 27B, and FIG. 27C are schematic diagrams illustrating inperspective views an optical waveguide comprising a non-inclinedlenticular surface, an inclined lenticular surface and an inclinedplanar surface;

FIG. 28 is a schematic diagram illustrating in perspective view anoptical waveguide comprising a non-inclined lenticular surface and firstand second inclined planar surfaces arranged on the non-inclinedlenticular surface;

FIG. 29A and FIG. 29B are schematic diagrams illustrating in perspectiveviews an optical waveguide comprising a non-inclined lenticular surfaceand first and second opposed inclined planar surfaces;

FIG. 29C is a schematic diagram illustrating in side view an opticalwaveguide comprising a non-inclined lenticular surface and first andsecond opposed inclined planar surfaces;

FIG. 30A is a schematic graph illustrating iso-luminance field-of-viewpolar plots for different positions across a backlight comprising anoptical waveguide of FIG. 29C and light turning film of FIGS. 11A-11Dwhen light is input into the first end of the optical waveguide;

FIG. 30B is a schematic graph illustrating iso-luminance field-of-viewpolar plots for different positions across a backlight comprising anoptical waveguide of FIG. 29C and light turning film of FIGS. 11A-11Dwhen light is input into the second end of the optical waveguide;

FIG. 31 is a schematic diagram illustrating in top view an automotivecabin comprising a display comprising the backlight comprising theoptical waveguide 1 of FIG. 29C,

FIG. 32 is a schematic diagram illustrating in side perspective view anoptical stack of a switchable privacy display device comprising aswitchable directional waveguide arranged to illuminate a spatial lightmodulator; and

FIG. 33 is a schematic diagram illustrating in side perspective view anoptical stack of a collimated display device comprising a switchabledirectional waveguide arranged to illuminate a spatial light modulator.

DETAILED DESCRIPTION

It would be desirable to provide a collimated backlight that provides arelatively narrow output cone angle for a display apparatus. In thepresent disclosure, collimated is used as an accepted term for narrowangle illumination from a display and/or backlight, for example fullwidth half maximum (FWHM) luminance cone angles of less than 40 degrees,and typically less than 30 degrees.

In comparison to conventional wide angle backlights, collimatedbacklights can provide high efficiency light output for head-onobservers, achieving increased luminance for a given power consumptionor reduced power consumption for a given luminance. Collimatedbacklights can also provide low off-axis image visibility for privacydisplay. It would further be desirable to provide a switchablecollimated backlight to provide a narrow angle output in a first mode ofoperation and a wide angle output in a second mode of operation. Inoperation, narrow angle output may be provided for a single head-onuser, while wide angle output may be provided for multiple displayusers.

The structure and operation of various switchable display devices willnow be described. In this description, common elements have commonreference numerals. It is noted that the disclosure relating to anyelement applies to each device in which the same or correspondingelement is provided. Accordingly, for brevity such disclosure is notrepeated.

FIG. 1 is a schematic diagram illustrating in side perspective view anoptical stack of a switchable privacy display 100 comprising aswitchable backlight 20 arranged to illuminate a switchable liquidcrystal retarder 300 and a spatial light modulator 48.

An optical waveguide 1 comprises first and second opposed light guidingsurfaces 6, 8 for guiding light along the optical waveguide 1 by totalinternal reflection. At least one light input end 2 is arranged betweenthe first and second light guiding surfaces 6, 8, wherein the lightinput end 2 extends in a lateral direction (y-axis direction).

Backlight 20 comprises the optical waveguide 1, rear reflector 3 andlight turning film 5. The second surface 8 of the optical waveguide 1 isarranged between the first surface 6 of the optical waveguide and thelight turning film 5; and the light turning film 5 is arranged toreceive light transmitted through the second surface 8 of the opticalwaveguide 1.

Backlight 20 further comprises light source 15 arranged to input lightinto the optical waveguide 1 at the first input end 2. Light source 15may comprise an array of light sources such as white light emittingdiodes (LEDs), the array of LEDs extending in the lateral direction. Atleast one further light source 17 that may be a further array of LEDs isarranged to input light into the optical waveguide 1 at the second inputend 4.

A control system comprising display controller 310, backlight controller314, first light source 15 driver 315 and second light source 17 driver317 may be arranged to control the luminous flux from the first andsecond light sources 15, 17.

Display apparatus 100 comprises the backlight 20, switchable liquidcrystal retarder stack 300 and spatial light modulator 48 to outputlight 400.

Spatial light modulator comprises input polariser 210; TFT substrate212; liquid crystal layer 214 that is pixelated with typically redpixels 220, green pixels 222 and blue pixels 224; color filter substrate216 and output polariser 218.

Switchable liquid crystal retarder stack 300 is arranged in series withthe backlight 20 and spatial light modulator 48. Stack 300 comprises aswitchable liquid crystal retarder 301 that comprises substrates 312,316 with transparent electrodes and alignment layers to providecontrollable alignment of a liquid crystal layer 314. Stack 300 furthercomprises an additional polariser 332 and compensation retarder 330, andvarious embodiments are described in U.S. Pat. No. 10,126,575 and inU.S. Patent Publ. No. 2019-0086706, both of which are hereinincorporated by reference in their entireties.

Switchable liquid crystal retarder stack 300 and spatial light modulator48 are arranged to receive light transmitted by the light turning film5. Diffuser 334 may be arranged to provide modification of output coneangle and further to minimise Moiré and mura artefacts.

The control system may further comprise switchable liquid crystalretarder stack 300 controller 312 and liquid crystal cell driver 315 tocontrol the voltage across the liquid crystal retarder 301. Controller312 is arranged to address voltage driver 350 such that the switchableliquid crystal retarder 301 is driven in a first alignment state whenthe display operates in a wide angle mode, and in a second alignmentstate when the display operates in a narrow angle privacy mode.

The structure of optical waveguide 1 will now be further described.

FIG. 2 is a schematic diagram illustrating in side perspective view anoptical waveguide 1 comprising a non-inclined lenticular surface 30 andinclined planar surfaces 32, 36; and FIGS. 3A-3B are schematic diagramsillustrating in perspective views an optical waveguide 1 comprising anon-inclined lenticular surface 30 and inclined planar surfaces 32, 36.

The at least one light input end of the optical waveguide 1 comprises afirst light input end 2 and a second light input end 4 facing the firstlight input end 2.

The first light guiding surface 6 of the optical waveguide 1 comprises aplanar surface.

The second light guiding surface 8 comprises (i) a plurality ofnon-inclined light extraction features comprising the lenticularsurfaces 30; and (ii) a plurality of inclined light extraction featurescomprising the inclined planar surfaces 32, 36 arranged in an array. Inthe present embodiments, the plurality of non-inclined light extractionfeatures comprise the lenticular surfaces 30, each comprising a curvedsurface that is extended in the longitudinal (parallel to x-axis)direction.

The structure of the second light guiding surface 8 will now bedescribed in further detail.

FIG. 4A is a schematic diagram illustrating in side perspective view anon-inclined lenticular surface 30 of an optical waveguide not in anintersection region 34.

The second light guiding surface 8 comprises a plurality of non-inclinedlight extraction features formed by lenticular surfaces 30 arranged inan array, each non-inclined light extraction feature 30 being elongateand extending in a longitudinal direction (parallel to the x-axisdirection). Each non-inclined light extraction feature 30 comprisessurface normal directions 130 a, 130 b, 130 c that vary in a plane 129orthogonal to the longitudinal direction and that have no component oftilt in the longitudinal direction.

FIG. 4B is a schematic diagram illustrating in side perspective view anon-inclined lenticular surface 30 of an optical waveguide in anintersection region 34.

The plurality of non-inclined lenticular surfaces 30 are intersected byinclined planar surfaces 32, 36 in an intersection region 34, and thewidth of the non-inclined light extraction feature 31 in theintersection region 34 varies in the longitudinal direction. In otherwords, in the present embodiment, each lenticular surface 30 is bisectedby the plane of the inclined planar surfaces 32, 36 adjacent theretosuch that its width reduces towards a cusp 37 between the planarsurfaces 32, 36.

FIG. 4C is a schematic diagram illustrating in side perspective view afirst inclined planar region 32 of an optical waveguide 1 in theintersection region 34.

The second light guiding surface 8 further comprises a plurality ofinclined planar surfaces 32 arranged in an array, each inclined planarsurface 32 comprising at least one surface normal direction 132 with atilt with tilt angle 133 with respect to the display normal direction130 that has a component in the longitudinal direction.

The plurality of inclined light extraction features comprises a firstplurality of inclined planar surfaces 32 shown shaded in FIG. 4C. Eachplanar surface 32 of the first plurality of inclined planar surfaces 32has a surface normal 132 that has a tilt angle 133 wherein the tilt ofthe surface normal 132 has a component in the longitudinal direction(parallel to x-axis) that is away from the first light input end 2.

The plurality of inclined light extraction features also comprises asecond plurality of inclined light extraction planar surfaces 36 shownshaded in FIG. 4E that is discussed further below. Each planar surface36 has a surface normal direction 136 that has a tilt angle 137 whereinthe tilt of the surface normal 137 has a component in the longitudinaldirection that is towards the first light input end 2.

The inclined planar surfaces 32, 36 are planar inclined light extractionfeatures. The planar surfaces 32, 36 may also have surface normaldirections that have no component in the lateral direction, that is thesurface normals 132, 136 may be arranged in the x-z plane.

FIG. 4D is a schematic diagram illustrating in side perspective view thetilted cross sectional profile 33 of the first inclined planar region ofan optical waveguide in the intersection region. The cross sectionalprofile 33 may comprise a triangular region for example. Advantageouslysuch a structure may be conveniently tooled as will be described below.

FIG. 4E is a schematic diagram illustrating in side perspective view asecond inclined planar surface 36 of the optical waveguide 1 in theintersection region 34; FIG. 4F is a schematic diagram illustrating inside view a non-inclined lenticular surface 30 of an optical waveguide1; and FIG. 4G is a schematic diagram illustrating in top view anon-inclined lenticular surface 30 of an optical waveguide and a firstinclined planar region of an optical waveguide in the intersectionregion. Features of the arrangements of FIGS. 2-4G not discussed infurther detail may be assumed to correspond to the features withequivalent reference numerals as discussed above, including anypotential variations in the features.

In an illustrative embodiment, the tilt angle 133 may be 2 degrees andthe tilt angle 137 may be 88 degrees. More generally in an opticalwaveguide 1 of the type illustrated in FIG. 2 , the tilt angle 133 inthe longitudinal direction of the first plurality of inclined planarsurfaces 32 may be between 0.25 degrees and 5 degrees, preferablybetween 0.5 degrees and 4 degrees and most preferably between 1 degreeand 3 degrees; and the tilt angle 137 in the longitudinal direction ofthe second plurality of inclined planar surfaces 36 may be between 80degrees and 90 degrees, and preferably between 85 degrees and 90degrees.

At least some of the plurality of non-inclined lenticular surfaces 30are intersected in intersection region 34 by at least one inclinedplanar surfaces 32, 36. FIGS. 14A-G hereinbelow illustrate a cuttingmethod for a tool to provide the surface 8 of the optical waveguide 1.During such tooling process, the planar surfaces 32, 36 may be cutthrough the lenticular surface 30, or the planar surfaces 32, 36 may becut through by the lenticular surface. The intersection regions 34 areprovided by the respective cut intersections.

In the present embodiments the arrays are two-dimensional. Illuminationmay be provided over a large area for an LCD backlight, advantageouslyachieving high uniformity.

The operation of the light extraction features will now be describedfurther for light input from the first input end 2.

FIG. 5A is a schematic diagram illustrating in side view operation of afirst inclined planar surface 32 of an optical waveguide 1 for on-axisillumination. Light ray 180 propagates by guiding between surfaces 6 and8. Light ray 180 has angle of incidence at surface 6, 8 graduallyreduced by the tapering of the planar surfaces 32 given by the tiltangle 133. Light rays that are at smaller angles of incidence than thecritical angle in the material of the optical waveguide 1 are extractedat angles close to grazing the first or second guiding surfaces 6, 8. Inoperation the tilt angle 133 of the planar surfaces 32 is arranged tonot provide alone sufficient light leakage from the optical waveguide 1;that is insufficient leakage would be present if the non-inclinedlenticular surface 30 were not present.

FIG. 5B is a schematic diagram illustrating in side view operation of anon-inclined lenticular structure for on-axis illumination. At eachreflection at the non-inclined lenticular surface 30, a deflection ofthe light ray 182 is provided that is out of plane of the paper in FIG.5B. The resultant ray thus sees a taper effect due to the inclinedsurface of the lenticular surface 30. Some reflections increase theangle of incidence while other reflections reduce the angle ofincidence. In operation the net ray angle of incidence change is smalland does not provide sufficient light leakage from the optical waveguide1; that is insufficient leakage would be present if the planar surfaces32 were not present.

The directionality of light extraction from the lenticular surface 30will now be further described for light rays incident at differentpositions across the lenticular surface 30 for light that has notundergone reflections at the planar surface 32.

FIG. 6A is a schematic diagram illustrating in top view operation of anon-inclined lenticular structure for off-axis illumination; FIG. 6B isa schematic diagram illustrating in end view operation of a non-inclinedlenticular structure for off-axis illumination; and FIG. 6C is aschematic diagram illustrating in side view operation of a non-inclinedlenticular structure for off-axis illumination.

Rays 184 a, 184 b, 184 c have locations of incidence 185 a, 185 b, 185 crespectively at the lenticular surface 30. In top view the rays 184 a,184 c are deflected by the inclined lenticular surface 30. In end view,the angle of reflection varies across the surface 30 while in side viewthe angle of reflection is unmodified. For each reflection, the rayangles are sufficiently above the critical angle that no light isextracted.

FIG. 6D is a schematic diagram illustrating in top view operation of aninclined planar feature for off-axis illumination; FIG. 6E is aschematic diagram illustrating in end view operation of an inclinedplanar feature for off-axis illumination; and FIG. 6F is a schematicdiagram illustrating in side view operation of an inclined planarfeature for off-axis illumination.

Rays 184 a, 184 b, 184 c have locations of incidence 185 a, 185 b, 185 crespectively at the planar surface 32. In top view and end view the rays184 a, 184 b, 184 c are slightly deflected by the inclined planarsurface 32. In side view the dominant effect of the planar surface 32can be visualised, the angle 187 b being smaller than the angle 187 a.Thus the tilt angle 133 of the planar surface 32 directs light rays 184b closer to the critical angle.

The combined effect of the planar surfaces 32 and non-inclinedlenticular surfaces 30 will now be described.

FIG. 6G is a schematic diagram illustrating in top view operation of anon-inclined lenticular structure for off-axis illumination afterincidence with an inclined planar surface 32; FIG. 6H is a schematicdiagram illustrating in end view operation of a non-inclined lenticularstructure for off-axis illumination after incidence with an inclinedplanar surface 32; and FIG. 6I is a schematic diagram illustrating inside view operation of a non-inclined lenticular structure for off-axisillumination after incidence with an inclined planar surface 32.

In comparison to the arrangement of FIGS. 6A-6C, the light rays 184 a-chave angles of incidence that have been reduced after reflection atplanar surface 32. Light rays 184 a, 184 b still remain above thecritical angle of incidence when incident on the lenticular surface 30.However, light ray 184 c is incident at an angle below the criticalangle and is extracted. The direction of extraction is inclined towardsthe longitudinal direction in comparison to the incident ray angle, asillustrated in FIG. 6G. In this manner, the planar surfaces 32 andlenticular surface 30 cooperate to extract light in directions close tothe longitudinal direction.

FIG. 7 is a schematic diagram illustrating in top view output of anoptical waveguide. Thus light cones comprising rays 188 a, 188 b, 188 care preferentially output from the lenticular surface 30, arising forlight travelling towards an inclined surface. Thus reflected ray bundles189 a-c are also provided from the oppositely tilted lenticular surface.

Features of the arrangements of FIGS. 5A-7 not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

FIG. 8A is a schematic graph illustrating simulated iso-luminancefield-of-view polar plot for the arrangement of FIG. 7 after incidenceonto a turning film 5 as illustrated in FIG. 8B which is a schematicdiagram illustrating in perspective view a light turning film comprisingplanar opposing faces 52, 54 of elongate prismatic elements 50. Theprofile is illustrated for a device arranged with input LEDs 15 arrangedalong a lower side of the waveguide 1 such that the direction parallelto the x-axis represents the elevation and the orthogonal directionrepresents the lateral direction for a viewer.

The field-of-view plots of the present disclosure illustrate thevariation of output luminance for longitudinal viewing angle againstlateral viewing angle. In the present illustrative embodiments, thesource 15 may be arranged at the lower edge of the display 100 and thesource 17 is arranged at the upper edge of the display 100. In thisarrangement, the horizontal viewing angle direction is in the lateraldirection (parallel to x-axis) and the vertical viewing angle directionis the longitudinal direction (parallel to y-axis).

In the longitudinal direction the light ray distribution is provided bylight at near grazing angles of incidence onto the light guiding surface8 and thus has a restricted cone angle. In the lateral viewing angledirection, the output luminance profile is determined by thedistribution of rays from the lenticular surface 30 as shown in FIG. 7 .

Advantageously a very narrow cone angle may be provided. Such anillumination profile may be used for high efficiency output to reducepower consumption or to increase output luminance for a given inputpower consumption. Further such a luminance profile may be used forprivacy display as will be described further hereinbelow.

It would be desirable to provide a wider angular profile of output thanthat provided in FIG. 8A.

FIG. 9A is a schematic diagram illustrating in top view operation of aninclined planar surface 36 for light from the second input end; FIG. 9Bis a schematic diagram illustrating in end view operation of an inclinedplanar feature for light from the second input end; and FIG. 9C is aschematic diagram illustrating in side view operation of an inclinedplanar feature for light from the second input end.

In comparison to FIG. 7 , light rays 190 a, 190 b incident on the planarsurface 36 are directed with a wide angular spread by refraction at theinterface. As will be shown further such an output provides a widespread of optical output. Advantageously an optical waveguide 1 with aswitchable wide angular range may be provided.

The extraction from the planar surfaces 36 is in proximity to the lightturning film 5 and not onto rear reflector 3. Efficiency of extractionis improved because of increased Fresnel reflections that wouldotherwise be present if the extraction was from the first guidingsurface 8. Advantageously efficiency of wide angle output is enhanced.

The operation of the backlight 20 of FIG. 1 comprising optical waveguide1 will now be further described.

FIG. 10 is a schematic diagram illustrating in side view a backlight 20comprising an optical waveguide comprising a non-inclined lenticularsurface 30 and inclined planar surfaces 32, 36, a rear reflector 3 and alight turning film 5. FIGS. 11A-11D are schematic diagrams illustratingin perspective views a light turning film 5 wherein each first prismaticface 40 comprises a planar surface and each second prismatic face 42comprises an undulating surface. Features of the arrangements of FIGS.10-11D not discussed in further detail may be assumed to correspond tothe features with equivalent reference numerals as discussed above,including any potential variations in the features.

Light turning film 5 comprises a first surface 55 that is typicallyplanar and a second surface facing the first surface 55 that comprisesan array of prismatic elements 50 that are elongate in the lateraldirection (parallel to y-axis). The prismatic elements 50 of the arrayof prismatic elements each comprise opposing first and second prismaticfaces 52, 54. Each first prismatic face 52 has a surface normal 152direction that is tilted by tilt angle 151 from the display normaldirection (z-axis direction) and has a component that is inclined in thelongitudinal direction (parallel to x-axis) towards the first input end2. Each second prismatic face 54 has a surface normal direction 154 thatis tilted by tilt angle 153 from the display normal direction (z-axisdirection) and has a component that is inclined in the longitudinaldirection away from the first input end 2.

When light source 15 is operated, light rays 172, 174 are input into thefirst input end 2 of the optical waveguide 1 and are guided by totalinternal reflection within the waveguide 1. As will be described furtherbelow, the pluralities of non-inclined lenticular surfaces 30 andinclined planar surfaces 32 are oriented to direct guided light rays 170through the first and second light guiding surfaces 6, 8 as output lightrays 172, 174. Planar surfaces 36 are hidden for light propagating fromthe first input end 2 and thus do not substantially contribute to outputlight.

The direction of output from the surface 8 is for light rays 172, 174that are near to the critical angle within the optical waveguide 1 andthus are typically close to grazing incidence from the surface 8 in air.

The backlight 20 further comprises a rear reflector 3 facing the firstlight guiding surface 6 that is arranged to reflect light rays 174 thatare transmitted through the first surface 6 of the optical waveguide 1.Light rays that pass through the surface 6 are incident on rearreflector 3 and are reflected back through the optical waveguide 1.

After output from the optical waveguide, output light rays 172, 174 fromthe optical waveguide 1 are input into the turning film 5 whereupon theyare refracted by the second prismatic face 54 and reflected by the firstprismatic face 52 by total internal reflection.

Light rays are reflected towards the display surface normal direction(parallel to z-axis), advantageously achieving high head-on luminance.

The undulations of second prismatic face 54 may provide some deflectionof light ray directions for rays 172, 174. For illustrative purposes,the surface 54 is shown as undulating in the x-z plane, orthogonal tothe lateral direction. However, the undulation is primarily in the x-yplane. Accordingly, rays are shown as diffusing in the longitudinaldirection, however such undulating facets illustrated in FIGS. 11A-11Dare arranged to provide diffusion primarily in the lateral direction.

Such deflections are provided by refraction at the undulating face 54and are thus relatively small. A small amount of light cone diffusionmay be provided for light rays 172, 174 by the undulating prismatic face54.

The present embodiments may provide for a single light source 15 at thefirst end 2. Advantageously a narrow output cone angle may be provided,achieving low off-axis luminance for privacy operation and high head-onefficiency as will be described further hereinbelow. Further a narrowbezel width may be achieved.

It maybe desirable to provide a second operating mode that provides awider viewing angle in comparison to the first operating mode.

A second light source 17 may be arranged to input input light into thesecond input end 4 of the optical waveguide 1. The pluralities ofnon-inclined light extraction features 30 and inclined planar surfaces32 are oriented to direct guided light rays 176 c within the opticalwaveguide 1 as guiding light rays. The planar surfaces 36 face thesecond input end 4 such that incident light rays 176 a, 176 b arerefracted as output light by the planar surfaces 36. As will bedescribed hereinbelow, the cone angle of light output from the planarsurface 36 for the transmitted light rays 176 a, 176 b may besubstantially greater than for light rays 172, 174.

The planar surfaces 36 are arranged on the same side of the opticalwaveguide 1 as the prismatic elements 50 of the light turning film 5.The output light is not reflected by the rear reflector 3 or outputthrough the first planar surface 6. In comparison to arrangements withplanar surfaces 36 arranged near to the reflector 3, in the presentembodiments Fresnel reflection losses are reduced so thatadvantageously, efficiency of extraction is improved in the second modeof operation.

Output light rays 176 a, 176 b from the optical waveguide 1 arerefracted by the first prismatic facet 52 and are reflected by thesecond prismatic face 54 by total internal reflection. In comparison tolight rays 172, 174, the light rays undergo a reflection at theundulating facets 54 rather than refraction. The effective optical powerof the reflecting surface is approximately three times the optical powerof the same surface for refracted light, and thus the undulation mayprovide substantial diffusion effect in comparison to that for lightrays 172, 174.

The planar surface 36 and undulating prismatic face 54 may achieve anoutput cone angle for light input from the second end by source 17 thatis substantially greater than the output cone angle achieved for lightinput from the input end 2 by source 15. Advantageously the output coneangle in wide angle mode may be substantially increased. Such abacklight 20 may provide a display 100 that may be conveniently viewedfrom a wide range of viewing angles.

The simulated luminance distribution of the illustrative embodiments ofFIGS. 2-6D will now be described for illumination by the light sources15 and the light sources 17.

FIG. 12A is a schematic graph illustrating iso-luminance field-of-viewpolar plots for different positions across a backlight comprising anoptical waveguide of FIG. 2 and light turning film of FIG. 8B when lightis input into the first end of the optical waveguide; and FIG. 12B is aschematic graph illustrating iso-luminance field-of-view polar plots fordifferent positions across a backlight comprising an optical waveguide 1of FIG. 2 and light turning film 5 of FIGS. 11A-11D when light is inputinto the first end of the optical waveguide.

Thus FIGS. 12A-12B illustrate simulated appearance for illumination ofthe optical waveguide 1 and turning film 5 of the present illustrativeembodiment by light source 15 where each contour represents a 20%luminance contour interval. The parameter X represents the relativedistance from the first input end 2, and is given by equation 1 where xis the distance from the input end 2 and L is the length of the opticalwaveguide 1, illustrated in FIG. 10 .

X=x/L  Eqn. 1

The luminance output profile is provided within approximately +/−20degree lateral viewing angle and +/−10 degree longitudinal viewing angleabout the display normal direction 130.

Advantageously such an illumination profile can achieve high efficiencyof illumination to a head-on user. Further, such a profile issubstantially uniform along the length of the optical waveguide 1,achieving high luminance uniformity. Such a profile can also be used toprovide the privacy mode operation of a privacy display as will bedescribed further below.

FIG. 12B differs from FIG. 12A in that substantially no increasedlateral diffusion is provided by the undulation on the face 45 of theturning film 5 in comparison to a planar face. Advantageously only asmall change in lateral field of view is provided by the undulations andprivacy performance is similar.

FIG. 12C is a schematic graph illustrating iso-luminance field-of-viewpolar plots for different positions across a backlight comprising anoptical waveguide of FIG. 2 and light turning film of FIG. 8B when lightis input into the second end of the optical waveguide; and FIG. 12D is aschematic graph illustrating iso-luminance field-of-view polar plots fordifferent positions across a backlight comprising an optical waveguideof FIG. 2 and light turning film of FIGS. 11A-11D when light is input bylight source 17 into the second input end 2 of the optical waveguide 1.In comparison to the output of FIG. 12B, the field of view is increasedto approximately +/−40 degrees in the lateral direction. Advantageouslythe display may be viewed from an increased range of lateral viewingdirections for a wide angle mode of operation.

FIG. 12D differs from FIG. 12C in that increased lateral diffusion isprovided by the undulation on the face 45 of the turning film 5 incomparison to a planar face. Advantageously the lateral field of view isincreased, and wide angle performance is improved.

The field-of-view profiles of FIGS. 12A-12D may be further expanded bymeans of diffuser 334 and diffusers arranged on other display surfacessuch as output polariser 218. The diffusers may be symmetric orasymmetric diffusers. For example, an asymmetric diffuser comprisingasymmetric surface relief microstructures for example may provideincreased cone angle in the longitudinal direction while providingsubstantially no change to the lateral angular profile. Advantageouslyviewing freedom in the longitudinal direction (that may typically be thevertical direction) may be expanded and privacy viewing angle in thelateral direction is substantially unmodified. Further Moiré beatingbetween the repeating cusps 37 of the optical waveguide 1, the lightturning film 5 and the pixels of the spatial light modulator 48, each ofwhich has major components in the longitudinal direction may beminimised.

The viewing angle profile for a single longitudinal viewing angle willnow be described.

FIG. 13A is a schematic graph illustrating angular luminance 70 againstlateral viewing angle 72 profiles 76, 78 for light input from first andsecond input ends 2, 4 of an optical waveguide 1; and FIG. 13B is aschematic graph illustrating normalised angular luminance profiles forlight input from first and second input ends 2, 4 of an opticalwaveguide 1. In privacy mode of operation, the profile 76 may beprovided for illumination by light source 15 while in wide angle modethe profile 78 may be provided for illumination by light source 17. Thefull width half maximum width for profile 78 is substantially greaterfor the profile 76. Further, for off-axis viewing positions, theluminance is substantially reduced.

Advantageously a switchable privacy display may be provided.

A method to provide a tool to be used to form the surface 8 of theoptical waveguide 1 will now be described.

FIGS. 14A-14E are schematic diagrams illustrating in perspective views amethod to form a tool for fabrication of a surface of an opticalwaveguide comprising non-inclined lenticular surface and inclined planarsurfaces.

In a first step as illustrated in FIG. 14A, a planar tool 60 blank isprovided. The tool blank may be for example copper or nickel. In asecond step as illustrated in FIG. 14B a diamond 62 with a curvedcutting face 63 is used to scribe cylindrical grooves 64 alignedparallel to the longitudinal direction in the surface of the tool 60,and a uniform lenticular surface is provided as illustrated in FIG. 14C.

In a third step a different diamond 66 with a planar cutting face 65that is inclined at angle 67 to the surface of the tool 60, where theangle 67 is typically the same as the angle 133 of the planar surfacenormal. The tool is used to scribe planar grooves in the lateraldirection, orthogonal to the longitudinal direction such that the toolcuts through the lenticular grooves 64 in the intersection region 34.

FIG. 14E illustrates an alternative cutting sequence. In a second step(not shown), linear facets 68 are cut in the lateral direction, and inthe third step lenticular grooves are cut through the linear facets 68by the diamond 62 with curved cutting face 63.

FIGS. 14F-14G are schematic diagrams illustrating in perspective viewstools for fabrication of a surface of an optical waveguide comprisingnon-inclined lenticular surface and inclined planar surfaces that havebeen cut as illustrated in FIGS. 14A-14E and comprising grooves 64 andplanar regions 61, 69. Features of the arrangements of FIGS. 14A-G notdiscussed in further detail may be assumed to correspond to the featureswith equivalent reference numerals as discussed above, including anypotential variations in the features.

Such a tool may be used for injection moulding, hot embossing or UVcasting of an optical waveguide 1 for example.

Advantageously the optical surface 6 is planar and may be convenientlyprovided by a support substrate or a planar injection mould. Further,high fidelity of features may be provided, achieving high efficiency ofextraction. The surface 8 may be provided on stiff substrates such as0.4 mm or greater thickness PMMA or PET substrates to provide lightguide plates (LGPs). Alternatively thin flexible substrates of thicknessless than 0.4 mm may be used to provide light guide films (LGFs) for usein flexible display.

It would be desirable to provide free-form shapes for display 100, forexample to provide curved corners or notches in the display area.

FIG. 15 is a schematic diagram illustrating in top view a curved inputshape optical waveguide 1. Input side 2 may be provided with linearregion 21 a and curved regions 21 c, 21 b by moulding or cutting of amoulded component. Advantageously a free form shape display may beprovided. Features of the arrangement of FIG. 15 not discussed infurther detail may be assumed to correspond to the features withequivalent reference numerals as discussed above, including anypotential variations in the features.

In typical privacy mode operation a display may be provided with arelatively narrow viewing angle in the lateral direction, and a widerviewing freedom in the longitudinal direction. It would be desirable tomaximise the viewing freedom in the longitudinal direction in order toenable display rotation about an axis in the lateral direction, that maytypically be the horizontal direction.

FIG. 16 is a schematic diagram illustrating in a first perspective viewa light turning film 5 comprising a planar opposing face 52 and a kinkedopposing face 54 comprising facets 56 a, 56 b that are inclined at angle57 to each other. FIG. 17 is a schematic diagram illustrating in asecond perspective view a light turning film 5 comprising a planaropposing face 52 and a kinked opposing face 54 and a planar outputsurface. Features of the arrangements of FIGS. 16-17 not discussed infurther detail may be assumed to correspond to the features withequivalent reference numerals as discussed above, including anypotential variations in the features.

The face 54 may comprise more than two facets or maybe a curved surface.In comparison to the undulating surface of FIGS. 11A-11D, the facets arearranged to provide diffusion primarily in the longitudinal direction.

In operation, light rays from the source 17 arranged at the second end 4are deflected by total internal reflection at face 54 of the turningfilm 5 and are spread in the longitudinal direction, as compared tospreading in the lateral direction by the undulating surface of FIGS.11A-11D.

FIG. 18A is a schematic graph illustrating iso-luminance field-of-viewpolar plots for different positions across a backlight 20 comprising anoptical waveguide 1 of FIG. 2 and light turning film of FIGS. 16-17 whenlight is input into the first end 2 of the optical waveguide 1; and FIG.18B is a schematic graph illustrating iso-luminance field-of-view polarplots for different positions across a backlight 20 comprising anoptical waveguide 1 of FIG. 2 and light turning film 5 of FIGS. 16-17when light is input into the second end 4 of the optical waveguide 1.

In the privacy mode of operation, a narrow field of view is provided toadvantageously achieve high efficiency and low privacy cross talklevels. In wide angle mode, the longitudinal direction field of view issubstantially increased in comparison to FIG. 12D for example.Advantageously the display may be visible from a wider range of viewingangles. Further the display may have substantially uniform illuminationprofile for both landscape and portrait viewing.

In the present embodiments, the total output for wide angle mode may beprovided by illumination from both light sources 15 and 17, and thus thecombined output may be a combination of FIGS. 18A and 18B in proportionsdetermined by the relative luminous flux of the driven sources 15, 17.Advantageously the output directionality may be tuned by control oflight sources 15, 17.

In a privacy mode of operation, the backlight 20 of the presentembodiments may provide stray light in viewing regions that may enable asnooper to perceive image data on a display, for example arising fromscatter and diffusion in the backlight 20. For example a snooper may beable to observe image data at luminances of greater than 0.5% of peakhead-on luminance, depending on ambient lighting conditions. A typicalnarrow angle backlight may achieve off-axis luminance of 2% or more foroff-axis viewing positions (for example lateral viewing positions of+−45 degrees). It would be desirable to minimise visibility of off-axisimages to a snooper.

FIG. 19A is a schematic diagram illustrating in top view the opticalstack of a privacy display comprising backlight 20 comprising an opticalwaveguide 1 illuminated by light source 15 at a first end 2 arranged toprovide privacy mode illumination, and a switchable liquid crystalretarder 300 driven in a first state of liquid crystal alignment.

Switchable liquid crystal retarders 300 for privacy display enhancementare described in U.S. Pat. No. 10,126,575 and in U.S. Patent Publ. No.2019-0086706, both of which are herein incorporated by reference intheir entireties.

In comparison to FIG. 1 , the spatial light modulator 48 is arrangedbetween the backlight 20 and the switchable liquid crystal retarder 300,however the performance would be expected to be substantially the samefor the two arrangements. Thus the display apparatus 100 furthercomprises at least one display polariser 218 or 210 arranged on a sideof the spatial light modulator 48; an additional polariser 318 arrangedon the same side of the spatial light modulator 48 as the displaypolariser 218; and a switchable liquid crystal retarder 330, 314comprising a layer of liquid crystal material 314 arranged between thedisplay polariser 218 and the additional polariser 318.

Returning to FIG. 1 , in operation in the second state for privacyoperation, the light source driver 315 is activated to drive the firstlight source 15. Further, the liquid crystal retarder 300 controller 312is operated to provide a narrow angle field of view. In other words,when a light source 15 is arranged to input light into the opticalwaveguide 1 at the first input end 2, a first voltage V₁ is appliedacross the switchable liquid crystal retarder 300 by means of electrodes311 a, 311 b such that the molecules of the liquid crystal layer 314have a first orientation arrangement.

In operation, unpolarised light rays 400, 402 from the backlight 20 arepolarised by spatial light modulator 48 input polariser 210 withelectric vector transmission direction 211. After modulation by pixellayer 214, output polariser 218 with electric vector transmissiondirection 219 provides polarisation states 360, 361 to rays 400, 402.Compensation retarder 300 and switchable liquid crystal layer 314 arearranged to provide substantially no change to the polarisation state362 for ray 400, but to provide a modified polarisation state 364 foroff-axis ray 402. Additional polariser 318 with electric vectortransmission direction 319 that is parallel to direction 219 transmitsray 400 but absorbs light on ray 402, substantially reducingtransmission for off-axis directions.

FIG. 19B is a schematic graph illustrating an iso-transmissionfield-of-view polar plot for a compensated switchable retarder in aprivacy mode of operation.

The orientation of the field-of-view profile may be arranged to providereduction of stray light luminance in privacy mode. For example, theoff-axis luminance may be reduced to less than 1% of the peak head-onluminance for a wide field of view by combination of the output lightcone of FIG. 18A and the transmission field-of-view profile of FIG. 19B.

Advantageously the output seen by a snooper is substantially reduced incomparison to displays that do not comprise the switchable liquidcrystal retarder, and enhanced privacy performance is achieved. Furtherthe head-on luminance is substantially unaffected, achieving highefficiency for the primary user.

It would be further desirable to provide a switchable wide field of viewfor a switchable privacy display.

FIG. 20A is a schematic diagram illustrating in top view the opticalstack of a privacy display comprising a backlight 20 comprising anoptical waveguide 1 illuminated by light source 17 at the second end 4arranged to provide wide angle illumination and a switchable liquidcrystal retarder 300 driven in a second state of liquid crystalalignment.

Returning to FIG. 1 , in operation in the second state for wide angleoperation, the light source driver 317 is activated to drive the secondlight source 17. Further, the liquid crystal retarder 300 controller 312is operated to provide a wide angle field of view. In other words, whena light source 17 is arranged to input light into the optical waveguide1 at the second input end 4, a second voltage V₂ different to the firstvoltage is applied by driver 350 across the switchable liquid crystalretarder 300 by means of electrodes 311 a, 311 b such that the moleculesof the liquid crystal layer 314 have a second orientation arrangement.

In operation, compensation retarder 300 and switchable liquid crystallayer 314 are arranged to provide substantially no change to thepolarisation state 362 for ray 400 and for off-axis ray 402. Additionalpolariser 318 with electric vector transmission direction 319 that isparallel to direction 219 transmits rays 400, 402, substantiallymaintaining luminance for off-axis directions.

FIG. 20B is a schematic graph illustrating an iso-transmissionfield-of-view polar plot for a switchable retarder in wide angle mode ofoperation. Advantageously the output seen by an off-axis viewer has highluminance and high efficiency operation is maintained. Further thehead-on luminance is substantially unaffected, achieving high efficiencyfor the primary user.

Features of the arrangements of FIGS. 19A-20B not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

Viewing of a privacy display will now be further described.

FIG. 21A is a schematic diagram illustrating in front perspective viewobservation of transmitted output light for a display operating inprivacy mode. Display 100 may be provided with white regions 603 andblack regions 601. A snooper 47 may observe an image on the display ifluminance difference between the observed regions 601, 603 can beperceived. In operation, primary user 45 observes a full luminanceimages by rays 400 to viewing locations 26 that may be optical windowsof a directional display. Snooper 47 observes reduced luminance rays 402in viewing locations 27 due to the reduction of luminance for off-axispositions from the optical waveguide and optional switchable liquidcrystal retarder 300.

FIG. 21B is a schematic diagram illustrating in front perspective viewsthe appearance of the display of FIG. 1 operating in privacy mode 1 withoff-axis luminance reduction. Thus upper viewing quadrants 530, 532,lower viewing quadrants 534, 536 and lateral viewing positions 526, 528provide reduced luminance, whereas up/down central viewing regions 522,520 and head-on viewing provides much higher luminance.

Advantageously a privacy display that has comfortable viewing forrotation about a lateral axis may be provided.

Another structure of optical waveguide 1 will now be described.

FIGS. 22-23 are schematic diagrams illustrating in side perspectiveviews an optical waveguide 1 comprising a non-inclined lenticularsurface 30 and inclined planar surfaces 32, 36 that do not intersect thenon-inclined lenticular surface 30; FIG. 24A is a schematic diagramillustrating in side perspective view a non-inclined lenticular surface30 of an optical waveguide 1; FIG. 24B is a schematic diagramillustrating in side perspective view a first inclined planar surface 32of an optical waveguide 1; and FIG. 24C is a schematic diagramillustrating in side perspective view a second inclined planar surface36 of an optical waveguide 1.

In simulation, such a structure provides similar field-of-view luminancecharacteristics to those illustrated above for FIG. 2 . In comparison tothe embodiment of FIG. 2 , such a structure may provide uniform elongatelenticular surfaces that are not cross-cut. Scatter at intersection ofthe cross-cut surfaces may be reduced. Advantageously privacyperformance may be improved.

Another structure of optical waveguide 1 will now be described.

FIG. 25 is a schematic diagram illustrating in side perspective view anoptical waveguide 1 wherein the second light guiding surface comprises anon-inclined elongate feature that is a prismatic feature comprising twosurfaces 80, 82 which each have a surface normal in a planeperpendicular to the lateral direction inclined at an opposite angle sothat the surface normal varies across the width of the elongateprismatic feature in the lateral direction. The second light guidingsurface also comprises first and second inclined intersecting planarsurfaces 32, 36 which form inclined light extraction features.

Features of the arrangements of FIGS. 22-25 not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

FIG. 26A is a schematic graph illustrating iso-luminance field-of-viewpolar plots for different positions across a backlight comprising anoptical waveguide of FIG. 25 and light turning film of FIGS. 11A-11Dwhen light is input into the first end of the optical waveguide; andFIG. 26B is a schematic graph illustrating iso-luminance field-of-viewpolar plots for different positions across a backlight comprising anoptical waveguide of FIG. 25 and light turning film 5 of FIGS. 11A-11Dwhen light is input into the second end of the optical waveguide.Advantageously the prismatic surfaces may be conveniently fabricatedwith planar surface diamonds. Further, the profile of output in privacymode can be enlarged, achieving increased uniformity for on-axisviewing.

Another structure of optical waveguide 1 will now be described.

FIGS. 27A-27C are schematic diagrams illustrating in perspective viewsan optical waveguide wherein the second guide surface comprises anon-inclined lenticular surface 30 that is a non-inclined lightextraction feature, and also an inclined lenticular surface 432 and aninclined planar surface 436 that are inclined light extraction features.That is, the inclined light extraction features comprise lenticularsurfaces 432 and planar surfaces 436 that are extended in thelongitudinal direction. Advantageously the same diamond that is used tocut the non-inclined lenticular surface 30 may be used to cut theinclined lenticular surfaces 432, reducing cost and complexity.

Another structure of optical waveguide 1 will now be described.

FIG. 28 is a schematic diagram illustrating in perspective view anoptical waveguide wherein the second guide surface comprises anon-inclined lenticular surface 30 that is a non-inclined lightextraction feature and first and second inclined planar surfaces 632,636 that are inclined light extraction features arranged on thenon-inclined lenticular surface 30. In comparison to FIG. 2 , the planarsurfaces 632, 636 may be arranged randomly across the light guidingsurface 8, advantageously reducing the appearance of Moiré.

It may be desirable to provide two different narrow angle illuminationdirections.

FIGS. 29A-29B are schematic diagrams illustrating in perspective viewsan optical waveguide wherein the second guide surface comprises anon-inclined lenticular surface 730 that is a non-inclined lightextraction feature and first and second opposed inclined planar surfaces732, 736 that are inclined light extraction features; and FIG. 29C is aschematic diagram illustrating the same optical waveguide in side view.

The tilt of the normal 731 to the non-inclined lenticular surface 730 inthe x-z plane is zero, that is there is no tilt in the longitudinaldirection.

First inclined planar surface 732 has surface normal 735 inclined in thelongitudinal direction towards the second input end 4 and secondinclined planar surface 736 has surface normal 737 inclined towards thefirst input end 2. The tilt angle 733, 739 of the normal directions 735,737 in the longitudinal direction of each of the first and secondpluralities of inclined planar surfaces 732, 736 is between 0.25 degreesand 5 degrees, preferably between 0.5 degrees and 4 degrees and mostpreferably between 1 degree and 3 degrees.

Features of the arrangements of FIGS. 27A-29C not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

FIG. 30A is a schematic graph illustrating iso-luminance field-of-viewpolar plots for different positions across a backlight comprising anoptical waveguide 1 of FIG. 29C and light turning film 5 of FIGS.11A-11D when light is input into the first end 2 of the opticalwaveguide 1 by light source 15; and FIG. 30B is a schematic graphillustrating iso-luminance field-of-view polar plots for differentpositions across a backlight comprising an optical waveguide of FIG. 29Cand light turning film 5 of FIGS. 11A-11D when light is input into thesecond end 2 of the optical waveguide 1.

Thus in a first mode of operation, provided by illuminating light source15 an off-axis illumination profile is provided with a 30 degrees offsetfrom the normal direction in the longitudinal direction. In a secondmode of operation an on-axis illumination profile may be provided byillumination of light source 17.

The relative offset of the two illumination profiles may be controlledby adjustment of the angles of the prism faces 52, 54 surface normaldirections of the light turning film 5.

Advantageously such a display may provide controllable illumination totwo separate users. The application of such a display to an automotivecabin environment will now be described.

FIG. 31 is a schematic diagram illustrating in top view an automotivecabin 552 of a vehicle 550 comprising a display 100 comprising thebacklight 20 comprising the optical waveguide 1 of FIG. 29C. Inoperation in a first mode, light source 15 is illuminated to provideillumination to passenger in light cone 554. Such a display may providelow luminance to the driver, and thus provide an entertainment functionfor example without distraction to the driver. The off-axis luminancemay be further reduced by means of the switchable liquid crystalretarder 300 of FIG. 1 .

For night-time operation, stray light may be reduced advantageouslyproviding increased safety from undesirable internal cabin illumination.Display size may be increased for low ambient illumination of theautomotive cabin.

In a second mode of operation the backlight 20 may be illuminatedalternatively or additionally by a second source 17 such that the drivermay perceive an image from the display.

The image displayed on the spatial light modulator may be timemultiplexed. In a first phase the light source 15 may be illuminated anda first image displayed on the spatial light modulator 48 that may be afast response liquid crystal display, for example capable of 100 Hz orhigher frame rates. In a second phase the light source 17 may beilluminated and a second image displayed of the spatial light modulator48. A first image may be supplied to the driver and a second image maybe supplied to the passenger. Advantageously the display 100 may providedifferent images to passenger and driver with full spatial resolutionfor each user at low cost and complexity.

Various display arrangements will now be described.

FIG. 32 is a schematic diagram illustrating in side perspective view anoptical stack of a switchable privacy display device comprising aswitchable directional waveguide arranged to illuminate a spatial lightmodulator. Thus the switchable liquid crystal retarder 300 of FIG. 1 maybe omitted. Advantageously thickness and cost is reduced. Such a displaymay conveniently provide high luminance or reduced power consumption aswell as night-time operation to reduce stray light to non-users.

FIG. 33 is a schematic diagram illustrating in side perspective view anoptical stack of a collimated display device comprising a non-switchabledirectional waveguide arranged to illuminate a spatial light modulator.Such a display omits the light source 17 and associated connections sohas a narrower bezel width in comparison to FIG. 1 and thus may bearranged in smaller form factors. The shape of the planar surfaces 32,36 and light turning film 5 may be adjusted to provide a larger coneangle than illustrated in FIG. 12A for example to achieve a desirablewide angle profile with privacy switching provided by the switchableliquid crystal retarder 300.

Features of the arrangements of FIGS. 33A-B not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

As may be used herein, the terms “substantially” and “approximately”provide an industry-accepted tolerance for its corresponding term and/orrelativity between items. Such an industry-accepted tolerance rangesfrom zero percent to ten percent and corresponds to, but is not limitedto, component values, angles, et cetera. Such relativity between itemsranges between approximately zero percent to ten percent.

While various embodiments in accordance with the principles disclosedherein have been described above, it should be understood that they havebeen presented by way of example only, and not limitation. Thus, thebreadth and scope of this disclosure should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with any claims and their equivalents issuing from thisdisclosure. Furthermore, the above advantages and features are providedin described embodiments, but shall not limit the application of suchissued claims to processes and structures accomplishing any or all ofthe above advantages.

Additionally, the section headings herein are provided for consistencywith the suggestions under 37 CFR 1.77 or otherwise to provideorganizational cues. These headings shall not limit or characterize theembodiment(s) set out in any claims that may issue from this disclosure.Specifically and by way of example, although the headings refer to a“Technical Field,” the claims should not be limited by the languagechosen under this heading to describe the so-called field. Further, adescription of a technology in the “Background” is not to be construedas an admission that certain technology is prior art to anyembodiment(s) in this disclosure. Neither is the “Summary” to beconsidered as a characterization of the embodiment(s) set forth inissued claims. Furthermore, any reference in this disclosure to“invention” in the singular should not be used to argue that there isonly a single point of novelty in this disclosure. Multiple embodimentsmay be set forth according to the limitations of the multiple claimsissuing from this disclosure, and such claims accordingly define theembodiment(s), and their equivalents, that are protected thereby. In allinstances, the scope of such claims shall be considered on their ownmerits in light of this disclosure, but should not be constrained by theheadings set forth herein.

1-25. (canceled)
 26. A backlight apparatus comprising: an opticalwaveguide comprising: first and second opposed light guiding surfacesfor guiding light along the optical waveguide by total internalreflection; and a light input end arranged between the first and secondlight guiding surfaces, wherein the light input end extends in a lateraldirection, at least one light source arranged to input light into theoptical waveguide at the light input end; and wherein the second lightguiding surface of the optical waveguide comprises: (i) a plurality ofnon-inclined light extraction features arranged in a two dimensionalarray, each non-inclined light extraction feature being elongate,extending in a longitudinal direction perpendicular to the lateraldirection, and having surface normal directions that vary in a planeorthogonal to the longitudinal direction and that have no component oftilt in the longitudinal direction; (ii) a first plurality of inclinedlight extraction features arranged in a two dimensional array, eachinclined light extraction feature of the first plurality of inclinedlight extraction features having a surface normal direction with a tiltthat has a component in the longitudinal direction that is away from thelight input end, wherein the component in the longitudinal direction ofthe tilt of the surface normal direction of the first plurality ofinclined light extraction features is between 0.25 degrees and 5degrees; and (iii) a second plurality of inclined light extractionfeatures arranged in a two dimensional array, each light extractionfeature of the second plurality of inclined light extraction featureshaving a surface normal direction that has a tilt with a component inthe longitudinal direction that is towards the light input end, whereinthe component in the longitudinal direction of the tilt of the surfacenormal direction of the second plurality of inclined light extractionfeatures is between 80 degrees and 90 degrees, the plurality ofnon-inclined light extraction features and the first and secondpluralities of inclined light extraction features being oriented todirect guided light through the first and second light guiding surfacesas output light.
 27. A backlight apparatus according to claim 26,wherein the light input end is a first light input end and the opticalwaveguide further comprises a second light input end facing the firstlight input end in the longitudinal direction, wherein the plurality ofnon-inclined light extraction features and the plurality of inclinedlight extraction features being oriented to direct guided light throughthe first and second light guiding surfaces as output light havingdifferent angular illumination profiles for light input through thefirst and second input ends.
 28. A backlight apparatus according toclaim 26, wherein the component in the longitudinal direction of thetilt of the surface normal direction of the first plurality of inclinedlight extraction features is between 0.5 degrees and 4 degrees, andpreferably between 1 degree and 3 degrees.
 29. A backlight apparatusaccording to claim 26, wherein the component in the longitudinaldirection of the tilt of the surface normal direction of the secondplurality of inclined light extraction features is between 85 degreesand 90 degrees.
 30. A backlight apparatus according to claim 26, whereinthe inclined light extraction features comprise planar inclined lightextraction features.
 31. A backlight apparatus according to claim 30,wherein the planar inclined light extraction features have surfacenormal directions that have no component in the lateral direction.
 32. Abacklight apparatus according to claim 26, wherein the inclined lightextraction features comprise lenticular surfaces that are extended inthe longitudinal direction.
 33. A backlight apparatus according to claim26, wherein the non-inclined light extraction features compriselenticular surfaces or elongate prismatic surfaces.
 34. A backlightapparatus according to claim 26, wherein the plurality of non-inclinedlight extraction features are intersected by the inclined lightextraction features.
 35. A backlight apparatus according to claim 26,wherein the first light guiding surface comprises a planar surface. 36.A backlight apparatus according to claim 27, further comprising at leastone further light source arranged to input light into the opticalwaveguide at the second input end.
 37. A backlight apparatus accordingto claim 26, further comprising a control system arranged to control theluminous flux from the at least one light source.
 38. A backlightapparatus according to claim 26, further comprising a light turning filmarranged to receive the output light, the light turning film comprisingan array of prismatic elements that are elongate in the lateraldirection.
 39. A backlight apparatus according to claim 38, wherein thesecond surface of the optical waveguide is arranged between the firstsurface of the optical waveguide and the light turning film, and thelight turning film is arranged to receive light transmitted through thesecond surface of the optical waveguide.
 40. A backlight apparatusaccording to claim 38, wherein the prismatic elements of the array ofprismatic elements each comprise opposing first and second prismaticfaces, wherein: each first prismatic face has a surface normal directionthat has a component that is inclined in the longitudinal directiontowards the first input end; and each second prismatic face has asurface normal direction that has a component that is inclined in thelongitudinal direction away from the first input end.
 41. A backlightapparatus according to claim 40, wherein each first prismatic facecomprises a planar surface and each second prismatic face comprises anundulating surface, wherein: when light is input into the first inputend of the optical waveguide, output light from the optical waveguide isrefracted by the second prismatic facet and is reflected by the firstprismatic face; and when light is input into the second input end of theoptical waveguide, output light from the optical waveguide is refractedby the first prismatic facet and is reflected by the second prismaticface.
 42. A backlight apparatus according to claim 26, furthercomprising a rear reflector facing the first light guiding surface thatis arranged to reflect light transmitted through the first surface ofthe optical waveguide.
 43. A display apparatus comprising the backlightapparatus of claim 38, further comprising a spatial light modulatorarranged to receive light from the light turning film.
 44. A displayapparatus according to claim 43, further comprising: at least onedisplay polariser arranged on a side of the spatial light modulator; anadditional polariser arranged on the same side of the spatial lightmodulator as the display polariser; and a switchable liquid crystalretarder comprising a layer of liquid crystal material arranged betweenthe display polariser and the additional polariser.
 45. A displayapparatus according to claim 44, wherein when a light source is arrangedto input light into the optical waveguide at the first input end, afirst voltage is applied across the switchable liquid crystal retarderand when a light source is arranged to input light into the opticalwaveguide at the second input end, a second voltage different to thefirst voltage is applied across the switchable liquid crystal retarder.46. A backlight apparatus according to claim 26, wherein the pluralityof non-inclined light extraction features are bisected by the plane ofthe inclined light extraction features adjacent thereto such that thewidth of the non-inclined light extraction features reduces towards acusp between the inclined light extraction features.